At first, actors Bing Crosby and Danny Kaye were just playing around on set when they dressed up in parts of their female counterparts' costumes and performed their musical number. But director Michael Curtiz found it so funny that he filmed it and worked it into the movie.

2. Crosby sang 'White Christmas' before this

That famous holiday tune dates back to 1942 --- more than a decade before this movie. Crosby first sang "White Christmas" in the movie "Holiday Inn" and again in the 1946 film "Blue Skies."

3. And that song actually may be a sad one

Composer Irving Berlin's 3-week-old son, Irving Berlin Jr., died on Christmas Day in 1928. Every Christmas, Berlin and his wife would visit their son's grave. Jody Rosen, author of "White Christmas: The Story of an American Song," told NPR that the song may have been Berlin's way of responding to his son's death.

4. Vera-Ellen doesn't really sing

It's widely debated whose singing voice we hear as Judy Haynes. Some say Trudy Stevens' voice doubled for all of Vera-Ellen's parts, while others say Rosemary Clooney sang as both Betty and Judy Haynes in "Sisters."

5. But she was a Radio City Rockette

Maybe Vera-Ellen couldn't sing, but she definitely could dance. The star began dancing when she was 10 years old and later became one of the youngest Rockettes.

6. Crosby was 25 years older than Clooney

While acting as one of the movie's romantic couples, Crosby was 51 years old. Clooney was 26.

7. The cast has family in the 'Star Trek' series

Crosby's granddaughter Denise Crosby appears in "Star Trek: The Next Generation." Clooney's son Miguel Ferrer was in "Star Trek III: The Search for Spock." Pretty out of this world, right?